To my shame I couldn't do it in one event.
I manage to generate either top and bottom or left and right.
Then the enemies fly to the center.
I must be getting old :(
x=random(ViewportLeft(0),ViewportRight(0))
y=choose(ViewportTop(0),ViewportBottom(0))
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x=choose(ViewportLeft(0),ViewportRight(0))
y=random(ViewportTop(0),ViewportBottom(0))